Exhaust-gas turbochargers are increasingly used for increasing power in internal combustion engines, in particular in motor vehicles. This is done ever more commonly with the aim of reducing the structural size and weight of the internal combustion engine while maintaining the same level of power or even achieving an increased level of power, and at the same time reducing consumption and thus CO2 emissions, in the context of ever more stringent legal regulations in this regard. The operating principle consists in utilizing the energy contained in the exhaust-gas flow to increase the pressure in the intake tract of the internal combustion engine and thereby realize improved charging of the combustion chamber with air oxygen and in order to thus be able to convert more fuel, gasoline or diesel, per combustion process, that is to say increase the power of the internal combustion engine.
For this purpose, an exhaust-gas turbocharger has a turbine, which is arranged in the exhaust-gas tract of the internal combustion engine and which has a turbine rotor driven by the exhaust-gas flow, and a compressor, which is arranged in the intake tract and which has a compressor rotor which builds up the pressure. The turbine rotor and compressor rotor are fastened rotationally conjointly to the opposite ends of a rotor shaft which is rotatably mounted in a bearing unit arranged between the turbine and the compressor. Thus, by means of the exhaust gas mass flow, the turbine wheel, and via the rotor shaft in turn the compressor wheel, are driven, and the exhaust gas energy is thus utilized for building up pressure in the intake tract.
Turbines and compressors are turbomachines and, owing to the laws of physics, have an optimum operating range in a manner respectively dependent on structural size and design, which optimum operating range is characterized by the mass throughput, the pressure ratio and the rotational speed of the respective rotor.
By contrast to this, the operation of an internal combustion engine in a motor vehicle is characterized by dynamic changes of the load and of the operating range.
To now be able to adapt the operating range of the exhaust-gas turbocharger to changing operating ranges of the internal combustion engine and thus ensure a desired response behavior as far as possible without noticeable decelerations (turbo lag), exhaust-gas turbochargers are equipped with additional functions, such as, for example, so-called variable turbine geometries (VTG) or wastegate devices (WG) on the exhaust-gas side and overrun air recirculation or blow-off devices on the air feed side. These serve for minimizing the inert behavior and thus the decelerated response behavior of the turbocharger and avoiding damaging operating states such as, for example, so-called compressor surging.
It is also known to use combinations of multiple turbochargers in a parallel or sequential arrangement or to use additional compressors which are operated mechanically or by electric motor, so-called supercharging blowers or superchargers, in order to cover the various operating conditions of the internal combustion engine, in order to efficiently increase the power in all rotational speed ranges and in particular during acceleration processes, and in particular to avoid the undesired turbo lag.
A supercharging device of said type, which has a conventional exhaust-gas turbocharger and an auxiliary compressor arranged in the fresh-air flow in series or in parallel with respect to the turbocharger compressor, which auxiliary compressor has a drive independent of the exhaust gas flow, for example an electric motor drive, is disclosed for example in DE 100 23 022 A1.
By contrast, in operating phases in which the power of the internal combustion engine is decreased quickly, it is the case, likewise owing to the inertia of the turbocharger, that an excess of compressor power exists, which can lead to compressor surging. In such operating states, exhaust gas is conducted, so as to bypass the turbine of the turbocharger, into the exhaust-gas tract, for example by means of a wastegate device, and already-compressed fresh air is blown off downstream of the compressor or is expanded via an overrun air recirculation device and recirculated into the intake region. The arrangement and functioning of an overrun air recirculation valve of said type are known, for example, from documents DE 28 23 067 C2 and DE 19712850A1.
In this way, the available energy is discharged, unutilized, into the surroundings, which has an adverse effect on the overall energy balance and thus on the efficiency of the internal combustion engine.